Instructions apply to Mac OS X 10.11 and higher.

SelectShow Message Sizein the drop-down menu.

Now, the size of a message appears under the date you received it.

Apple Mail with View highlighted on the menu bar

If you want to sort your email by size, selectSort Byin the View menu and chooseSize.

Why Would You Want to Know the Message Size?

You might want to know the message size to determine whether a message has attachments or images in it.

Mail View menu with Show Message Size highlighted

Apple Mail inbox with the message size displayed

Apple Mail with Sort By and Size highlighted